Proximity of iron pnictide superconductors to a Mott insulating state and the complex phase diagrams of (Ba₁₋ₓNaₓ)Fe₂As₂

High-Tc superconductivity in the cuprates occurs at the crossover from a correlated Mott insulating state to a weaker correlated Fermi liquid as a function of doping. The Fe-pnictides were initially thought to be weakly correlated. However, we have recently shown that KFe2As2 is in fact a highly correlated metal and that these correlations are even further enhanced in Rb- and CsFe2As2 . Whereas the correlations in the cuprates result from a large value of the Hubbard U, recent works have stressed the particular relevance of Hund’s coupling J in the pnictides. Our data may be interpreted in terms of a close proximity to an orbital- selective Mott transition. We now have good thermodynamic data covering both the hole and electron doping sides of the BaFe2As2 system and we will discuss how these correlations are modified by doping and uniaxial pressure.
We have also re-examined in greater detail the underdoped region of Ba1-xNaxFe2As2 using thermodynamic measurements (heat capacity and thermal expansion). Besides the magnetic C2 phase, we found a small region of C4 symmetry inside the SDW state similar to that of Ba1-xKxFe2As2 . However, no subsequent re-entrance to the C2 phase was observed at lower temperature in Ba1-xNaxFe2As2 and its phase diagram is shown to be considerably more complex than previously reported, containing 9 different phases. ... mehrDifferences and similarities between the Ba1-xNaxFe2As2 and Ba1-xKxFe2As2 systems will be discussed and we will show how these new phases interact with superconductivity.